Multiple factors are involved in onset of acne and folliculitis, but the most important factor is the growth of some Gram-positive anaerobic bacteria such as an Propionibacterium acnes and Staphylococcus in pilosebaceous ducts.
Conventionally, as the main treatment method of acne and folliculitis, an external antibacterial drug such as nadifloxacin has been frequently used for treatment of mild to moderate, while an oral antibacterial drug such as minocycline or roxithromycin has been frequently used for moderate to severe. However, such external antibacterial drugs were not sufficiently effective, and the oral drugs had severe problems of adverse effects during long-term administration and an increase in resistant bacteria.
Normally, the efficacy of an external therapy with an antibacterial drug is largely dependent not only on the antimicrobial activity of the active ingredient, but also on the thickness of the stratum corneum in the affected area and the penetration efficiency of the antibacterial drug into the corneum. Thus, it is important that an antimicrobial preparation is compatible with the skin and that it should contain an antibacterial drug that is completely dissolved uniformly and superior in stability, and also highly penetrable.
One reason for the difficulty of complete healing only with an external antibacterial drug is that it is difficult to deliver the active ingredient to the region under the stratum corneum or the hair follicle, where skin indigenous bacteria such as the Propionibacterium acnes and Staphylococcus generally proliferate.
As an example of a preparation that allows rapid penetration of the active ingredient deep into the skin, a skin external preparation containing a pyridonecarboxylic acid derivative, N-methylpyrrolidone, oleic acid, and propylene glycol (Patent Document 1) is proposed.
However, N-methylpyrrolidone is included in the Class 2 solvents, of which the blending amount should be restricted, in the guideline “Q3C Impurities: Residual Solvents” of medicines revised by the International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) in October 2002, and thus, use of N-methylpyrrolidone is preferably avoided wherever possible (Nonpatent Document 1). Use of N-methylpyrrolidone should also be avoided from the viewpoint of safety.    Patent Document 1: Japanese Laid-open Patent Publication No. 2002-356426    Nonpatent Document 1: PDE for N-Methylpyrrolidone (NMP) Q3C (M), Residual Solvents, ICH Harmonised Tripartite Guideline